Effects of Route of Administration and Dietary Hemoglobin on the Liver Mitochondrial a-Glycerophosphate Dehydrogenase Response to Various Thyroxine Analogs

Abstract

The rat liver a-glycerophosphate dehydrogenase (GPD) response to thyroid hormone was used to assay the relative activities of several thyroxine analogs which were either injected or fed to rats receiving a purified diet [plus or minus]10% hemoglobin (Hb). Both the route of administration and the presence of the antithyrotoxic substance (Hb) in the diet markedly affected the relative activity values. For example, triac and triprop were more active when fed in either the basal or Hb diets than when injected into animals receiving the same diets, whereas the converse was found for T4 [thyroxine] and T3 [triiodothyronine]. Because of this divergence, triac was as active as T3 when both analogs were fed in the basal diet. Although dietary Hb inhibited the activity of all analogs, it inhibited certain ones more than others. For example, the activity of T4 fed in the Hb diet was only 6% of that fed in the basal diet, whereas the activities of oral T3 and triac were reduced to 30 and 64%, respectively, by the addition of Hb to the diet. The maximum GPD responses were appreciably higher for analogs fed in the basal diet, and the "apparent" straight-line segments of the dose-response curves often had slightly different slopes.